The present invention relates generally to wireless receivers, and more particularly to receivers configured to operate in multiple multiplexing schemes.
The explosive growth in telecommunication and multimedia applications demands flexible, efficient, high performance receivers. Most current digital receivers focus on detecting a transmitted signals for a particular modulation or multiplexing scheme. That is, the receivers are xe2x80x9chard-wiredxe2x80x9d for only one scheme, or multiple schemes are implemented with redundant hardware. Because many different communication standards exist, a flexible receiver architecture is necessary. If the receiver can be configured in real time to adapt to different multiplexing schemes, a user can operate the receiver in different geographical areas, or select a particular communication service provider at will.
In addition, for mobile communication devices, which are becoming increasingly popular, fast-fading is a problem. Fast-fading is exhibited as rapid temporal fluctuations in the power of the received signal. Consequently, when the mobile user is moving, the resulting Doppler spread substantially degrades the receiver""s performance due to errors in channel state estimation. Multi-path interference is another serious problem. In a multi-path channel, the original transmitted signal reflects from obstacles in the channel path, such as buildings, high terrain, and mountains. The receiver has to contend with several copies of the signal, the copies having different delays. Actually, from each multi-path signal""s point of view, other multi-path signals can be regarded as interference and they need to be suppressed by the processing gain.
Fading, and the associated Doppler spread, as well as multi-path interference will continue to be the central issues for the next generation of wireless communication devices. It is desired to provide receivers that minimize these channel impairments.
In the prior art, a partial solution has been proposed. The solution is based on a direct sequence spread spectrum (DSSS) reception algorithm as described by Sayeed et al. in xe2x80x9cJoint Multipath-Doppler Diversity in Mobile Wireless Communications,xe2x80x9d IEEE, Transactions on Communications, vol. 47, pp. 123-132, January 1999. This receiver is designed specifically for CDMA multiplexing, and will not work for other multiplexing schemes.
As shown in FIG. 1, their solution uses a DSSS receiver 100 for coherently processing a received signal 101. If channel coefficients are not available, and only their power is known, then incoherent processing may well be used. The receiver 100 uses short time Fourier transforms (STFT q1 and STFT q0) 102. Channel responses, i.e., the channel coefficients, 103 are applied to the transformed signals in a combiner 104. The resulting signals are provided to an adder 105 to produce a signed real part of the received signal (Sgn(Re(*)) 106. In Sayeed""s receiver the matched filters are configured as conventional RAKE receivers, and therefore can only process a single multiplexing scheme, i.e., CDMA, as stated above.
Their algorithm combines time and frequency diversities to achieve higher processing gains, especially for fast fading channels. In fact, their STFF-based algorithm is a special case of filter bank-based transmultiplexers as described by Akansu et al. in xe2x80x9cOrthogonal Transmultiplexers in Communication: A Review,xe2x80x9dIEEE Trans. Signal Processing, vol. 46, no. 4, pp. 979-995, April 1998.
However, configuring these prior art receiving techniques for multiple multiplexing schemes is still a problem because of the design of the matched filters and other hard-wired restrictions.
The invention provides an architecture for a digital radio receiver adaptable to multiple multiplexing schemes of operation. The present architecture uses software to configure the receiver in real time. The software executes on a generic receiver hardware platform. The software configurable receiver can adapt to multiple multiplexing schemes such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiplexing (OFDM), Multi Carrier CDMA (MC-CDMA), and the like.
By changing receiver configuration parameters at will, using software, the receiver according to the present invention can support multi-scheme and multi-band operations. This provides a solution for a cost effective receiver in the presence of the existence of multiple standards in the communication industry.
Instead of using multiple filter banks to perform multiple multiplexing schemes as in the prior art, the present invention uses a sampled-STFT-decomposition-based architecture for different multiplexing schemes. By using a software-controlled method, the present invention enables a digital receiver to operate in multiple multiplexing schemes by changing configuration parameters. The receiver also has channel equalization capabilities by combining channel estimates with decomposed samples.
More particularly, in a digital radio receiver, transmitted symbols are recovered from a received signal that includes time-shifted and frequency-shifted copies of a transmitted signal. A channel estimator extracts channel characteristics from a training signal. Receiver data and software for execution in a processor are stored in a memory connected to the processor.
A parameter controller generates receiver configuration parameters from the channel characteristics and the receiver data. The received signal is decomposed into a matrix of samples according to the receiver configuration parameters to adapt the receiver to multiple multiplexing schemes. An inner product is formed from samples and the channel characteristics. The symbols are recovered from the inner product.